Pressure swing adsorption processes are well-known for the separation of gas mixtures that contain components with different adsorbing characteristics. In such processes the gas mixture is subjected to adsorption in an adsorption zone containing adsorbent at an elevated adsorption pressure and the thus loaded adsorbent is subsequently freed from the adsorbed gas components by depressurizing the adsorption zone to a lower desorption pressure, thereby achieving a desorption of said components. The adsorption zone, thus regenerated is repressured again and the adsorption can be resumed. To ensure a steady flow of purified gas a number of adsorption zones are used. An issue that arises during the desorption relates to the gas that is withdrawn from the adsorption zone in the desorption stage. This gas may have a composition similar to the gas mixture to be treated and may therefore be valuable. Such gas is therefore generally not vented, but is used to repressure another adsorption zone that has been previously reduced in pressure to the desorption pressure. After such use there may still be a relatively large amount of valuable gas left. A solution could be to recover this gas in a so-called residue gas vessel, by equalizing the pressures in the adsorption zone and the residue gas vessel. It will be evident that due to the equalization of these pressures the prevailing pressure in the residue gas vessel may amount up to a significant superatmospheric value.
The prior art has recognized the problem of re-using the gas contained in the adsorption zone before depressuring. In U.S. Pat. No. 3,142,547 it is proposed to allow such gas to flow into a pressure equalization zone, to depressure the adsorption zone to the desired desorption pressure, subsequently to pass the gas stored in the pressure equalization zone through the adsorption zone in order to wash the latter zone, and to dispose of the gas passed through the adsorption zone. The disadvantage of the known process is that though the gas is used to wash the adsorption zone, the valuable components in the gas are disposed of, thereby lowering the efficiency of the pressure swing adsorption process. U.S. Pat. Nos. 3,430,418 and 3,564,816 describe pressure swing adsorption processes in which at least 4 adsorption zones are present and in which the gas of each adsorption zone is used to repressure the other adsorption zones. After such repressuring any excess pressure in the first adsorption zone is disposed of by venting the excess gas. The disadvantage of these processes resides in the requirement that at least four expensive adsorption zones must be present. Further, the processes do not provide for any use of the gas that is disposed of. An even more expensive process is proposed in U.S. Pat. No. 3,986,849 where a process is described requiring at least seven adsorption stages and at least three pressure equalization stages.
All the above U.S. Patents recognize that the pressure swing adsorption process is suitable for the purification of hydrogen. The hydrogen-containing gas mixture subjected to pressure swing adsorption is suitably derived from a steam reforming process employing a hydrocarbonaceous feedstock, such as natural gas or other light hydrocarbons. This process is well-known. Reference is made in this respect to U.S. Pat. No. 3,810,975 and German Patent Application No. 2,911,669. Said German application describes a steam reforming process in which the produced hydrogen-containing gas mixture is subjected to a pressure swing adsorption process and a residue gas from the pressure swing adsorption process is recycled to the steam reforming process to be combusted and provide part of the heat for the endothermic steam reforming process.
It is an object of the present invention to provide a pressure swing adsorption process that allows for further use of residue gas.
It is another object of the invention to provide for a pressure swing process wherein the efficiency of the purification is improved.
Still another object of the invention is to provide an inexpensive alternative to the presence of a significant number of adsorption stages.
A further object of the invention is the provision of a process for the manufacture of hydrogen with an improved efficiency.